Energy and Reactions

Energy: the ability to promote change or do work
Two forms
- Kinetic energy: associated with movement
- Potential Energy: due to structure or location
Heat (thermal energy): kinetic energy associated with random movement of atoms or molecules
Chemical energy: the energy in molecular bonds; a form of potential energy

First Law of Thermodynamics: “Law of conservation of energy”; energy cannot be created or destroyed, but can be transformed from one type to another
Second Law of Thermodynamics: transfer of energy from one form to another increases the entropy (degree of disorder) of a system
- As entropy increases, less energy is available for organisms to use to promote change

Change in free energy determines direction of chemical reactions
Total energy = Usable energy + Unusable energy
- H = G + TS
- Energy transformations involve an increase in entropy (disorder that cannot be harnessed to do work)
- Free energy (G): the amount of energy available to do work
- Also called Gibbs free energy
ΔG = Δ H – T Δ S
Exergonic = spontaneous
- ΔG<0 (negative free energy change)
- Energy is released by reaction
Endergonic = not spontaneous
- ΔG>0 (positive free energy change)
- Requires addition of energy to drive reaction

Spontaneous reactions: occur without input of additional energy
Not necessarily fast, can be slow
- Breakdown of sucrose to CO2 and H2O is spontaneous, but will take a long time for \n sugar in a sugar bowl to break down
Key factor is the free energy change – if ΔG is negative, then process is exergonic and spontaneous

Reactions in a closed system eventually reach equilibrium and then do no work
Cells are not in equilibrium; they are open systems experiencing a constant flow of materials
- Metabolism is never at equilibrium
- A catabolic pathway in a cell releases free energy in a series of reactions
Closed and open hydroelectric systems can serve as analogies

ATP powers cellular work by coupling exergonic reactions to endergonic reactions
A cell does three main kinds of work:
- Chemical
- Transport
- Mechanical
To do work, cells manage energy resources by energy coupling the use of an exergonic process to drive an endergonic one
Most energy coupling in cells is mediated by ATP
In the cell, the energy from the exergonic reaction of ATP hydrolysis can be used to drive an endergonic reaction
This release of energy comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselves

ΔG = -7.3 kcal/mole
Reaction favors formation of products
The energy liberated is used to drive a variety of cellular processes
The reactions will be spontaneous if the net free energy change for both processes is negative

Activation energy: Initial input of energy to start reaction
Allows molecules to get close enough to cause bond rearrangement
Can now achieve transition where bonds are stretched
Common ways to overcome activation energy
- Large amounts of heat (living cells can’t do that because heating it would denature the enzyme)
- Using enzymes to lower activation energy